NASA scientists found that Earth's center of mass wanders only about half as far each year as previously thought — and revealed what's driving the wobble
Every GPS position on the planet is calculated against one invisible point: Earth’s center of mass. Satellite navigation treats that point as fixed. It isn’t. Snowmelt, ocean currents, and shifting air drag it back and forth by several millimeters every year, and scientists just found that the swing is about half the size they thought it was eight years ago.
Old estimates couldn’t agree
Earth’s center of mass isn’t the same thing as its geometric center, the exact middle point of the planet if it were a perfectly smooth ball.
Because water, ice, and air keep sloshing between hemispheres, the real center of mass shifts around that geometric middle by several millimeters over the course of a year.
GPS, satellite altimetry, and elevation surveys all use that point as their reference, so a wobble in it can distort measurements everywhere else.
The trouble is that nobody could measure the wobble precisely enough. The last two international estimates, made in 2017 and 2023, differed by 0.27 inches (7 millimeters), about the height of three stacked nickels.
That gap was almost as large as the motion the estimates were supposed to be measuring.
Old satellites, a new fix
A new study, led by geoscientist Donald Argus at NASA’s Jet Propulsion Laboratory, set out to narrow that gap.
Researchers from the University of Nevada, the University of Montana, and the GFZ Helmholtz Centre for Geosciences also contributed.
Satellites naturally orbit Earth’s center of mass, pulled there by gravity, so tiny changes in the distance between a satellite and a ground station show how that center is moving.
That idea isn’t new. Two dense metal satellites called LAGEOS, launched in 1976 and 1992, were built for precision geodetic measurements.
Each weighs about 900 pounds, is studded with reflective prisms, and gets tracked by laser from ground stations in more than 20 countries.
The catch is that those ground stations aren’t spread evenly around the globe, which leaves gaps in the coverage. Argus’s team filled those gaps two ways.
They added GPS tracking and orbital data from several low-orbiting satellites, giving the calculation a much wider set of targets.
They also built in how the weight of water and ice bends Earth’s crust, since that bending drags the ground stations themselves slightly out of position.
Snow and rain move the center
Once the new method was running, the team split the seasonal push into three sources: oceans, air, and water sitting on land, which includes ice, snow, rivers, lakes, soil moisture, and groundwater.
Snow piling up across North America and Eurasia peaks every March and nudges Earth’s center of mass about 0.12 inches (3 millimeters) toward the North Pole.
Land ice counts here too – satellite gravity data recently caught Antarctica gaining ice for a couple of unusual years, before the continent’s long-term melting resumed.
A month after the March snow peak, rain adds roughly 2,400 gigatons of water to the Amazon River basin – 2.6 trillion tons of water – and the center of mass swings roughly 0.09 inches (2.2 millimeters) toward South America.
A wet Amazon season can move enough water to show up in sea level data too. Monsoon rains over Southeast Asia add a smaller push in November, peaking at 600 billion tons.
The same satellite gravity records also underlie recent work on how monsoon timing reshapes Himalayan glaciers year to year.
Oceans swell in early fall
Between August and October, meltwater and rain pour into the oceans, and Earth’s center of mass shifts toward the South Pacific.
The Pacific is so enormous that mass changes there far outweigh what happens in every other ocean, though the Mediterranean, Red, North, Baltic, and Barents seas each add their own small nudge.
Air has weight too, and cold air is denser than warm air.
The team used an atmospheric model from the European Centre for Medium-Range Weather Forecasts.
Dense winter air shifts the balance toward Arabia, Asia, and northern Africa around December 21. Around June 21, it shifts toward South America and southern Africa.
Satellites confirm the shift
The team checked its results against the twin GRACE-FO satellites, which have measured monthly shifts in Earth’s gravity since 2018.
The satellites detect changes as they pass over heavy masses, such as swollen river basins. The two records lined up.
“We’re now estimating the size of the movement of Earth’s mass center back and forth each year to be about half of what we believed it to be eight years ago,” Argus said.
“Our findings suggest that the mass of Earth’s water and air moving between the hemisphere is smaller than previously thought.”
Nobody yet knows whether the smaller estimate will hold as more tracking data comes in. Future global estimates will show whether the 7-millimeter gap has truly narrowed.
Why tiny shifts matter
Sea level tracking depends on this same center-of-mass reference point.
Meanwhile, projects like Starlink satellites are being pressed into service to map other invisible shifts in the space around Earth.
Felix Landerer, a coauthor on the study at JPL, put the stakes in plain terms.
“While these movements might appear tiny, our modern world relies on extremely accurate positioning measurements,” he said.
“By unraveling and understanding the mechanisms that change reference systems, we can build better reference systems that ultimately benefit mapping and navigation – from global shipping logistics to precision agriculture.”
The full study was published in the journal Geophysical Journal International.
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